The present invention relates to a cellulosic article and a method for manufacturing same. The present invention is particularly suitable for tubular food casings, such as sausage casings which may be provided in a shirred form.
It is known that cellulosic articles may be made by a variety of procedures. For example, cellulose with or without chemical modifications, may be put into solution with a solvent, e.g. by dispersion or by dissolution, and then shaped into an article followed by solvent removal (with or without chemical modification of the cellulose) to solidify or set the shape of the formed cellulosic article. Examples of known processes for production of cellulosic articles are the viscose, cuprammonium, N-methyl-morpholine-n-oxide, zinc chloride and cellulose carbamate processes as described e.g. in U.S. Pat. Nos. 1,601,686; 2,651,582; 4,145,532; 4,426,228; 4,781,931; 4,789,006; 4,867,204; and 4,999,149, the teachings of which are all hereby incorporated by reference. The formed article may have a variety of shapes including a filament, bead, sheet or film. It is contemplated that the present invention may utilize any known method of producing a cellulosic article of any shape. With further respect to the present invention, although the article may be of any shape, preferred are films, generally having a thickness of 10 mils (254 microns) or less. Both planar, spherical, cylindrical and tubular articles are contemplated with seamless tubular articles being preferred and tubular films being especially preferred.
The present invention is particularly useful with respect to the manufacture and use of food casings. Food casings used in the processed food industry are generally thin-walled tubing of various diameters prepared from regenerated cellulose, cellulose derivatives, and the like.
In general, cellulosic food casings have multifunctional uses in that they may be employed as molding containers during the processing of the food product encased therein and also serve as a protective wrapping for the finished product. In the sausage meat industry, the preparation of various types of sausages such as frankfurters in a variety of sizes usually involves removing the casing from about the processed meat prior to final packaging. These sausages from which casing is usually removed are generally formed and processed in nonfiber-reinforced (nonfibrous) cellulose casing. However, larger diameter sausages such as salami are frequently sold with the casing left on. These sausages are usually formed and processed in fiber-reinforced (fibrous) cellulosic casing.
The starting material in the manufacture of cellulosic food casings is high quality, relatively pure cellulose pulp (either cotton or wood), most typically in sheet form. In present commercial manufacture of nonfibrous cellulose sausage casings, regenerated cellulose is generally made using the well known viscose process whereby viscose is typically extruded through an annular die into a coagulating and regenerating bath to produce a tube of regenerated cellulose. (Rayon filaments or threads may be similarly made by extrusion through a spinning nozzle according to well known methods.) This tube is subsequently washed, plasticized e.g. with glycerine, and dried e.g. by inflation under substantial air pressure. A typical commercial viscose process is described below which utilizes cellulosic sheet starting materials having a suitable density between about 0.8-0.9 gm/cc.
This relatively pure cellulose is typically converted to alkali cellulose by steeping in a sodium hydroxide solution. Cellulose absorbs the sodium hydroxide and the fibers swell and open. The degree of steeping is preferably held to the minimum amount necessary to ensure uniform distribution of the sodium hydroxide on the cellulose. A steeping bath temperature of about 19.degree.-30.degree. C. is preferred, and a suitable sodium hydroxide concentration in the steeping bath is about 17-20 wt. %.
In a typical steeping apparatus there is no forced circulation of caustic between the cellulose sheets, so it is important that the rate of filling the apparatus with caustic (fill rate) be such that the caustic reaches every portion of the sheets. The cellulose sheets are typically held in place in the steeping chamber by a support frame, and a typical steep time in commercial practice is 50-60 minutes.
After steeping, the caustic is drained and excess absorbed sodium hydroxide solution is pressed out, typically by a hydraulic ram. A typical alkali cellulose composition is about 13-18% caustic, 30-35% cellulose and the remainder water (by wt.). The percent caustic and cellulose in the alkali cellulose is controlled by the well-known press weight ratio. This ratio is the weight of the wet cake after pressing divided by the weight of the original cellulose used. A typical press ratio is about 2.6-3.2. After the press out, the alkali cellulose is shredded, i.e. the fibers in the sheet are pulled apart so that during xanthation the carbon disulphide contacts all portions of the alkali cellulose. There is an optimum shredding time for each system which can only be determined by testing. Typical shredding time is about 40-90 minutes. Heat is generated during the shredding step and the temperature may, for example, be controlled by means of a cooling water jacket around the shredder, preferably in the range of 25.degree.-35.degree. C.
During a succeeding, preferred aging step, an oxidative process is initiated which breaks the cellulose molecular chains thereby reducing the average degree of polymerization which will in turn reduce the viscosity of the viscose to be produced. During the aging step the shredded alkali cellulose is preferably maintained in covered vessels to prevent drying.
The conversion of alkali cellulose to cellulose xanthate is accomplished by placing the shredded and aged alkali cellulose in a closed reactor known as a baratte and adding carbon disulphide which vaporizes and reacts with the alkali cellulose to form cellulose xanthate. The amount of carbon disulphide used to achieve the desired conversion to cellulose xanthate is typically equal in weight to about 26-38% of the bone dry weight cellulose in the alkali cellulose, and preferably only enough to produce cellulose xanthate with acceptable filtration characteristics.
The length of time required for the xanthation reaction (conversion of alkali cellulose to cellulose xanthate) depends on the reaction temperature and the quantity of the carbon disulphide. Variations in such parameters as the quantity of carbon disulphide used as well as the temperature, and pressure during xanthation is determined by the desired degree of xanthation. The percent total sulphur is directly related to the amount of carbon disulphide introduced, including xanthate and by-product sulphur. In general, xanthation reaction conditions are varied to ensure that adequate conversion is achieved by reaching a total sulphur content greater than about 1.1 wt. %. Typically, there is about 0.4-1.5% by wt. sulphur in the by-products admixed with cellulose xanthate.
The purpose of converting alkali cellulose to cellulose xanthate is to enable dissolution of the cellulose in a dilute solution of sodium hydroxide, e.g. 3.6-5.0 wt. %. This is the so-called viscose formation or "vissolving" step, in which sodium hydroxide is absorbed onto the cellulose xanthate molecule which becomes highly swollen and dissolves over a finite time period. This step is preferably accelerated by cooling and agitation. Sufficient cooling is preferably provided to maintain the mixture at about 10.degree. C. or less. The quality of the solution is typically determined by measuring the filterability of the viscose e.g. by rate of clogging or throughput through a filter such as a cloth filter. The viscose is allowed to ripen and deaerate, is filtered under controlled temperature and vacuum. During ripening, reactions occur which result in a more uniform distribution of the xanthate group on the cellulose and a gradual decomposition of the xanthate molecule which progressively reduces its ability to remain dissolved, and increases the ease of viscose-cellulose regeneration.
Viscose is essentially a solution of cellulose xanthate in an aqueous solution of sodium hydroxide. Viscose is aged (by controlling time and temperature) to promote a more uniformed distribution of xanthate groups across the cellulose chains. This aging (also termed "ripening") is controlled to facilitate gelation or coagulation. If the desired product is a tube, the tubular form is obtained by forcing the viscose through a restricted opening, for example, an annular gap. The diameter and gap width of the opening, as well as the rate at which the viscose is pumped through, are designed in a manner well known to those skilled in the art for both non-reinforced and fiber-reinforced products such that a tubular film casing of specific wall thickness and diameter is formed from the viscose.
The extruded viscose casing is converted (coagulated and regenerated) to cellulose in the extrusion bath by action of a mixture of acid and salt, for example, sulphuric acid and sodium sulphate. A typical bath contains about 7-18% sulfuric acid by weight, and the bath temperature may be about 30.degree.-56.degree. C.
The cellulose casing emerging from the acid/salt bath is preferably passed through several dilute acid baths. The purpose of these baths is to ensure completion of the regeneration. During regeneration, gases (such as H.sub.2 S and CS.sub.2) are released through both the inner and outer surfaces of the casing, and means must be provided for removing these gases from the casing. After the casing has been thoroughly regenerated and the salt removed, it is preferably passed through a series of heated water baths to wash out residual sulfur by-products.
Cellulose articles for use as sausage casing require plasticization e.g. with moisture and/or polyols such as glycerine. Without such plasticization the casings are too brittle for commercial use. Typically, a softener such as glycerine is added in the final water bath by way of a dip tub, and in a quantity of about 11-16% based on the bone dry cellulose weight (for typical nonfibrous casing). The regenerated cellulose casings are also typically dried e.g. by inflation with heated air. After drying, the casing is wound on reels and subsequently shirred on high-speed shirring machines, such as those described in U.S. Pat. Nos. 2,984,574, 3,451,827, 3,454,981, 3,454,982, 3,461,484, 3,988,804 and 4,818,551. In the shirring process, lengths of from about 40 to about 200 feet of casing are typically compacted (shirred) into tubular sticks of between about 4 and about 30 inches. The shirred casing sticks are packaged and provided to the meat processor who typically places the sticks over a stuffing horn and causes the casing sticks to be deshirred at extremely high speeds while stuffing the deshirred casing with a foodstuff such as meat emulsion. The encased foodstuff can be subsequently cooked and the casing removed, from e.g. meat processed therein, with high-speed peeling machines.
For fibrous casing, a process of manufacture similar to that for nonfibrous casing is employed, however, the viscose is extruded onto one or both sides of a tube which is usually formed by folding a web of paper so that the opposing side edges overlap. In production of fibrous casing the viscose impregnates the paper tube where it is coagulated and regenerated to produce a fiber-reinforced tube of regenerated cellulose. The paper provides fiber reinforcement which is generally utilized in tubular casing having diameters of about 40 mm or more in order to provide dimensional stability, particularly during stuffing with meat emulsion. Production of both nonfibrous and fibrous casing is well-known in the art and the present invention may utilize such well known processes modified as disclosed herein.
Cellulosic casings are typically humidified to a level sufficient to allow the casing to be shirred without undue breakage from brittleness. A humectant may be employed to moderate the rate of moisture retention and casing swelling to produce a casing which during the shirring operation has sufficient flexibility without undue swelling or sticking of the casing to the shirring mandrel. Typically, a lubricant such as an oil will also be used to facilitate passage of the casing through the shirring equipment e.g. over a shirring mandrel.
It has been useful to lubricate and internally humidify cellulose casings during the shirring process by spraying a mist of water and a lubricant through the shirring mandrel. This is an economical, fast and convenient way to lubricate and/or humidify the casing to increase the flexibility of the casing and facilitate high speed shirring without undue detrimental sticking, tearing or breakage of the casing.
Cellulosic food casings suitable for use in the present invention may have a moisture content of less than about 100 wt. % based upon the weight of bone dry cellulose (BDC). The term "bone dry cellulose" as used herein refers to cellulose such as regenerated cellulose and/or paper which has been dried by heating the cellulose in a convection oven at 160.degree. C. for one hour to remove water moisture. In the formation of cellulosic casing e.g. by the viscose process, regenerated cellulose prior to drying forms what is known as gel stock casing having a high moisture content in excess of 100 wt. % BDC. This gel stock casing is unsuitable for stuffing with food such as meat emulsion, e.g. to form sausages, because it has insufficient strength to maintain control of stuffing diameter and prevent casing failure due to bursting while under normal stuffing pressure. Gel stock casing is typically dried to a moisture level well below 100 wt. % (BDC) which causes the regenerated cellulose to become more dense with increased intermolecular bonding (increased hydrogen bonding). The moisture level of this dried casing may be adjusted, e.g. by remoisturization, to facilitate stuffing. Such remoisturization or moisture adjustment to a specific level, for nonfibrous casing, is typically to a level with a range of from about 5 to about 40 wt. % BDC. Small diameter nonfibrous casing, prior to shirring, will have been dried to a typical moisture content of about 10-20 wt. % BDC, and such small diameter nonfibrous casing when shirred will have a moisture content that has been adjusted to between about 20 to 40 wt. % BDC.
For fibrous casing, casing is commercially produced having a moisture content ranging from about 4 wt. % BDC to about 70 wt. % BDC. Typically, fiber-reinforced casing having a moisture level between about 4 to about 25 wt. % BDC will be soaked prior to stuffing by a food processor. Premoisturized, ready-to-stuff, fibrous casing is also commercialized. Premoisturized fibrous casing which does not require additional soaking or moisturization will typically have a moisture content of from about 26 to about 70 wt. % BDC.
In the formation of skinless (casing removed) frankfurters, sausage proteins coagulate, particularly at the sausage surface, to produce a secondary skin and allow formation of a liquid layer between this formed skin and the casing as described in U.S. Pat. No. 1,631,723 (Freund). In the art the term "skinless frankfurter" is understood to mean that the casing is or is intended to be removed by the producer and that such casing may be removed because of formation of the secondary "skin" of coagulated proteins on the surface of the frankfurter. This secondary skin forms the outer surface of the so called "skinless frankfurters". Skin formation is known to be produced by various means including the traditional smoke curing with gaseous smoke, low temperature drying, application of acids such as citric acid, acetic acid or acidic liquid smoke or combinations thereof. Desirably, this secondary skin will be smooth and cover the surface of the frankfurter. Formation of a liquid layer between the casing and the frankfurter skin relates to the meat emulsion formulation, percent relative humidity during the cooking environment, subsequent showering and steam application to the chilled frankfurter and presence of any peeling aid coatings at the casing/frankfurter interface.
In present commercial production of tubular cellulose casings it would be desirable to improve process efficiency, productivity and costs with respect to certain process steps. For example, during the cellulose regeneration step, as described above, sulfur-containing gases and water vapor accumulate inside the regenerating tube. These waste gases must be removed, and this is commonly done by slitting the casing walls at intervals so the waste gases may be vented. However, the slit sections of cellulose tube must be ultimately removed and the adjoining sections spliced together. This procedure is time consuming, labor intensive and results in product waste because the slit sections of tubing must be discarded. So there is a long-standing need in cellulose casing production to reduce the required frequency of puncturing/slitting. The potential advantages would include higher extrusion speed (if the time interval between puncturing/slitting is to remain constant) or longer intervals between puncturing/slitting if the extrusion speed is to remain constant.
One limitation of the prior art cellulose tube manufacturing system is the time, equipment expense and material cost required to add softener to the casing. Most commonly this involves the additions of between about 10 and 20% glycerine to nonreinforced cellulose tubing and between about 15 and 35% glycerine to fiber-reinforced cellulosic casing (all on a total weight basis of casing). It is certainly desirable to reduce or even eliminate the need for this softener addition step. This has not heretofore been possible because low softener content reduces the flexibility of the cellulose tube wall, thereby causing excessive breakage due to inherent distortions during the shirring and compression steps to form the as-sold shirred stick. Low softener content may also result in excessive breakage when, after a storage period of typically at least ten weeks before use, the stick is deshirred by the food processor and stuffed with food, e.g. frankfurter emulsion.
Another disadvantage of the softener requirement is that unabsorbed softener is a substantially noncompressible liquid which resists compression during shirring. Moreover, the softener tends to make the shirred and compressed stick expand or grow immediately after manufacture, so either the sticks must be allowed time for longitudinal stabilization before packaging for shipment to food processors, or placed in cartons with unrestricted space at the ends for longitudinal growth. The latter is undesirable because the longitudinally slidable sticks may tend to bow and break. It will also be apparent that the softener adds weight to the shirred stick shipping carton, and that the casing manufacturer wishes to provide food processing customers with sticks having the highest useable inflatable casing length per unit length shirred stick, often referred to as "pack ratio".
Olefinic oxide polymers such as poly(ethylene oxide) are known materials having a wide variety of suggested uses. Various grades of a commercially available poly(ethylene oxide) sold under the trademark POLYOX.RTM. have been suggested as useful as adhesives, flocculants, and filler retention and drainage aids in the manufacture of paper and paperboard products. Other suggested uses include, thickeners for paints, drag-reducing additives for water used in fire fighting, lubricants and thickeners for personal care products such as toothpastes and shaving preparations, and also as dispersant, binders and rheology control agents in a variety of applications. Poly(ethylene oxide) has also been used in water soluble packaging films and to increase water retention in soil. Other functions and uses are disclosed in the brochure POLYOX.RTM. WATER-SOLUBLE RESINS (Union Carbide Chemicals & Plastics Technology Corporation, 1990).
Poly(ethylene oxide) is known as an additive to thermoplastic films to promote biodegradability. It is susceptible to severe auto-oxidative degradation and loss of viscosity in aqueous solutions. According to the Handbook of Water-Soluble Gums and Resins by Robert C. Davidsons, (published by McGraw-Hill Book Company, 1980) the degradation mechanism involves the formation of hydroperoxides that decompose and cause cleavage of the polymer chain. The rate of degradation is increased by heat, ultraviolet light, strong acids, or certain transition metal ions.
The present invention ameliorates the above noted limitations or disadvantages in various embodiments as further described below.
One object of this present invention is to provide an improved cellulosic tube with lower polyhydric alcohol softener content than heretofore used.
A further object is to provide a shirred cellulosic tube without polyhydric alcohol softener but still having high coherency and low breakage rate.
Another object is to provide a shirred cellulosic tube stick with higher pack efficiency than heretofore achieved under equivalent shirring and compression conditions.
Still another object is to provide a method for manufacturing a cellulosic casing wherein less waste gases are produced during the cellulose regeneration step.
A further object is to provide a sausage casing having improved peelability under difficult peeling conditions.
Another object is to provide a casing which either before drying (gel stock) or after drying (semi-finished) has a faster rate of absorption i.e. it takes up and holds greater amounts of water or liquid based (especially aqueous) coatings such as migrating or nonmigrating colorants, flavorants, antimycotics, liquid smokes, skin forming agents, preservatives, peeling aids or meat adhesion promoters in a shorter period of time.
Still another object is to provide a cellulosic casing having an olefinic oxide polymer such as poly(ethylene oxide) which has an average molecular weight of at least about 70,000 uniformly incorporated with cellulose.
An additional object is to provide a cellulosic casing having a plurality of layers or sections wherein an olefinic oxide polymer is uniformly incorporated or dispersed throughout at least one layer or section (preferably the innermost layer of a tubular article) but optionally not in all layers or sections.
Yet another object is to provide a casing having a combination of high packing efficiency and a high bore size.
A further object is to provide a casing having a combination of high packing efficiency and high pack ratio.
Another object is to provide a method for manufacturing a regenerated cellulose casing using a viscose process having a faster rate of regeneration.
Still another object is to provide a method for manufacturing a casing which fulfills any of the above objects.
An additional object is to provide a method for manufacturing a shirred cellulosic casing stick with high pack ratio, high coherency and low breakage.
These and other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims. It is not necessary that each and every object listed above be found in all embodiments of the invention. It is sufficient that the invention may be advantageously employed relative to the prior art.